The specific aims of this project involve clinical studies in normal pregnancies and pregnancies complicated by fetal growth restriction. The studies cover two aspects: 1) improvements in the classification of clinical severity using new fetal surveillance techniques and 2) studies of amino acid transport and metabolism using stable isotope methodology. The role of direct measurements of umbilical venous blood flow compared to arterial velocimetry measurements and fetal blood chemistry is examined around the framework of the classification of clinical severity. Similarly, the additional information acquired from single fetal organ velocimetry studies of the ductus venosus, fetal hepatic veins, fetal femoral artery, fetal middle cerebral artery are also compared to fetal umbilical arterial velocimetry and fetal chemistries. The studies of in vivo amino acid placental transport and metabolism is a follow up on our observations that fetal amino acid concentrations are significantly reduced in fetal growth restricted pregnancies. A non steady state study utilizing stable isoptomers of essential and non essential amino acids in pregnant women scheduled for cordocentesis examines in vivo transport for different groups of amino acids: a) to define differences in their relative rates of transport in vivo and b) to test whether amino acid transport is reduced in FGR pregnancies compared to normal pregnancies for all amino acids or a specific subset. Steady state stable isotope methodology is also used to test whether the fetal/maternal enrichment ratios for stable isoptomers of amino acids infused in the maternal circulation is lower in FGR pregnancies reflecting decreased transport and increased protein turnover. The relationship of maternal concentration to umbilical uptake for normal and FGR pregnancies is also investigated utilizing data on umbilical arteriovenous concentration differences at the time of cesarean section. All metabolic data are compared to the classification of clinical severity in FGR pregnancies using criteria we have previously published.